1. Field of the Invention
The present invention relates to an improved die for use in blow molding, preferably with a resin that is a so-called "engineering plastic", for producing a parison having an elliptical annular profile, in a cross-sectional view, which parison is preferably blow molded to produce a hollow double wall type plastic article having a basically concave, convex or bowl shape or a panel shape with a hollow double wall.
2. Description of the Related Art
A typical conventional die for use in blow molding comprises a vertically extending tubular housing and an integrated combination of a mandrel and a core, mounted in the housing to thus define a peripheral space gap therebetween providing an annular and axial melt passage. The die has a feed hole for feeding the melt from an extruder, which hole communicates with the melt passage. The melt passage forms die lips defined by the core and the housing at a lower free end of the die, through which a parison of the melt is provided for the blow molding. The mandrel has the feed hole formed such that it has an inlet opening at a top end portion of the mandrel and is branched to form a pair of diametrically opposite outlet openings in the top end portion at a peripheral surface thereof.
The mandrel has a pair of grooves symmetrical relative to its axis, each having a curved upper edge of an upwardly convex form and a horizontally straight edge with a substantially flat bottom surface therebetween, in a developed front view of the mandrel, so that it has an arc or semi-circle type profile. The paired grooves are connected to and communicate with each other at their opposite lower ends, while the feed outlet openings communicate with the grooves at top and central points thereof, respectively. Each groove has an upper side surface arced in a cross-sectional view, which surface is defined between the upper edge and the bottom surface, and has a cross-sectional area decreasing in a direction from the top and central point toward each lower end. The mandrel has symmetrical isolated lands or islands formed in the flat grooves (which may be called "seas") immediately below the feed outlet openings at the top and central points, respectively, so that the melt from each feed outlet opening is divided into two streams at the island in the groove.
The mandrel has a circumferential flat land upwardly adjacent to the flat grooves at the upper edge thereof. This flat land and the paired islands are sealingly fitted to an inner peripheral surface of the housing.
The mandrel has multi-staged straight dams extending radially and peripherally for a downward melt flow with circumferential straight grooves defined therebetween. The melt passage is defined between a portion of a peripheral surface of the mandrel forming the grooves, the islands and the dams, and a corresponding portion of the inner peripheral surface of the housing.
The entire melt passage has an upper zone having a circular periphery, a lower elliptical periphery zone and an intermediate transitional zone therebetween. The lower elliptical zone is defined by a lower section of the mandrel and the core integrated therewith, and a lower section of the housing therebetween, and terminates at die lips which have an elliptical annular profile and open at a free lower end of the die.
The lower housing section and the core are replaceable parts to be connected to the upper housing section and the mandrel, respectively, and they are designed in respect of their profiles and sizes so that the melt is ejected from the die lips to form a parison having a desired profile and size for use in producing blow molded articles having a specific profile and size.
With the above conventional die, there are the following problems:
The die is required to have a pressure loss of the melt from the extruder to the die lips distributed over the entire periphery of the melt passage at the die lips as uniformly as possible, in order to obtain a parison having a desired profile and size as expected at a predetermined melt extrusion rate. In order to obtain such a desired parison, the upper circular melt passage zone must be provided so as to have a pressure loss of the melt from the extruder to a lower end of the upper zone distributed over the entire circular periphery of the upper zone at the lower end thereof as uniformly as possible. However, according to the conventional die, a degree of the uniformity of the pressure loss distribution is improved as the number of the multi-staged dams is increased, but it is noted that the increased number results in enlarging the die in respect of a vertical length thereof, and increasing the pressure loss per se. These results require an increased power extruder and an enlarged die, and thus are economically disadvantageous.
Further, even if the uniform distribution of the pressure loss in the upper melt passage zone is attained, this does not ensure that the pressure loss at the die lips is distributed uniformly, except for a case where the die lip profile is circular. Therefore, in a case of an elliptical die lip profile, it is required to design the core and the lower housing part so that they render the melt from the upper melt passage zone to be ejected from the die lips through the intermediate and lower melt passage zones with a uniform pressure loss at the die lips. Such a design is difficult and troublesome, and is required every time a desired parison is changed in respect of its profile and size. Further, an operation of exchanging the lower housing part is also troublesome, since such a part is generally heavy, relative to the core.
In a blow molding operation using a parison having a specific profile and size, there may often occur cases where a resin of a product is changed to another kind of material or the resin is the same but a color of the resin is changed. If such a resin change or resin color change is required, it is desired that an old resin remaining in the die be completely discharged by extruding a new resin melt through the die with amounts of the new resin being wasted and, time and load required in this transitional operation should be as low as possible. However, with the conventional die, the old melt is apt to remain in various local places in the melt passage of the die, particularly a place immediately below the islands, the lower ends of the flat grooves connected to each other, the straight grooves, and local portions of the inner peripheral surface of the housing with relatively large radial space gaps from the mandrel. In this regard, it is desired to improve the die so that such melt remaining portions in the die are diminished and/or the old melt can be completely discharged with decreased amounts of the wasted new melt, and the time and load required in such a transitional operation.